This invention relates to a digital diagnostic apparatus using digital X-ray image data for medical diagnosis.
In general, a digital X-ray diagnostic apparatus comprises an image intensifier to convert a transmitting X-ray image from an object (patient) into a visible light image, a television camera to convert the light image into an analog television video signal, an analog-to-digital converter to convert the analog video signal into a digital video signal, and a digital memory to store the digital video signal. The digital video signal stored in the memory can be read out at a high speed when necessary, thus providing an image of the object to facilitate diagnosis.
In recent years, a digital fluorography apparatus capable of removing image information (for example, bones) unnecessary for diagnosis through a data subtraction process, has been attracting attention.
The digital fluorographic apparatus includes an X-ray photographing device, an image processing unit for processing an X-ray image information signal from the X-ray photographing device, and a display unit for displaying the processed image information. The X-ray photographing device includes an X-ray tube, an image intensifier (I.I.) for converting a X-ray image of an object into a visible light image, and a TV camera for converting the light image to a television signal. The image processing unit is adapted to convert the television signal representing the X-ray image of the object from the TV camera to a digital information signal; to subtractively combine an image information signal obtained before the injection of an X-ray contrast medium into the object with an image information signal obtained after the injection of the contrast medium, and to convert the subtraction image information signal to an analog information signal to permit it to be displayed on a TV monitor.
In the X-ray photographing device, the dynamic range (effective video signal level range) of the TV camera is narrower than that of the image intensifier. There is also a limit on the dynamic range of an A/D converter in the image processing unit. The maximum allowable input level of the A/D converter is set at the maximum allowable input level of the TV camera.
If the signal level of the optical image of the image intensifier deviates from the effective video signal level range of the TV camera, an optimum image is not obtained. For example, halation occurs on the display screen with respect to the white level of the video signal, and the system fails to obtain a detailed image of the corresponding area. To display an optimum X-ray image on the screen of the TV monitor, it is important to set the apparatus at the optimum X-ray generation conditions. In the prior art apparatus, before the execution of subtraction image processing, test X-rays are directed to the object, and the video signal obtained from the X-ray TV camera is sampled. The X-ray generation conditions are then optimally controlled based on the level of the video signal thus sampled. In this way, feedback control is obtained.
Normally, tissues of a human body are not uniform in the regions of interest to which the digital fluorographic apparatus is applied. Even if a system is designed so that the X-ray conditions are automatically set, it would be difficult to obtain optimum video signal levels over the whole display screen. For this reason, corrective operations must be performed, for example material of a low transmittance is inserted as a corrective filter into a site of a high X-ray transmittance, to obtain a uniform transmittance over the whole display screen.
However, since the X-ray transmittance of each region of interest of the body is not known, it is necessary for the operator to repeatedly perform the corrective operations while observing resultant images. This causes the body to be exposed to a greater dosage of X-rays than is desirable.